ASYMMETRICAL TOPOLOGY OF DIETHER-TYPE AND TETRAETHER-TYPE POLAR LIPIDS IN MEMBRANES OF METHANOBACTERIUM-THERMOAUTOTROPHICUM CELLS

We investigated the distribution of diether polar lipids between the i nner and outer leaflets of the membrane of Methanobacterium thermoauto trophicum comparing the orientation of tetraether polar lipids, which constitute a monolayer in the same membrane. Three kinds of reactions were employed for intact cells or protoplasts and unsealed membrane fr agments prepared from the organism: glycosidase digestion for glycolip ids, NaIO4 oxidation for glycolipids and inositol lipids, and trinitro phenylation for aminophospholipids. The results indicated that (a) mos t gentiobiose residues of both diether and tetraether polar lipids wer e exposed on the outside of the cells; (b) serine and inositol residue s of both diether and tetraether polar lipids were mainly oriented to the cytoplasmic surface of the membrane; and (c) approximately 80% of archaetidylethanolamine (diether type) was distributed in the outer le aflet of the membrane bilayer, while only 25% of the ethanolamine resi due of gentiobiosyl caldarchaetidylethanolamine (tetraether type) was oriented to the outer surface of the membrane. These results, except f or ethanolamine lipids, are consistent with the hypothesis that the te traether polar lipids are synthesized from the corresponding diether p olar lipid precursors that have been already substituted by polar grou ps in the membrane by head-to-head condensation without rearrangement of lipids.